27.7-keV Level inI129and Some Remarks about Its Use in Mössbauer Experiments

The properties of the first excited state of ${\mathrm{I}}^{129}$ have been studied with a mass-separated ${\mathrm{Te}}^{129m}$ source. The energy of this state, as determined from the conversion-electron spectrum, is $E_{\gamma }=27.72\mathrm{}\pm 0.06$ keV. This differs considerably from the often quoted value of 26.8 keV, but it agrees very well with the value recently published by Bemis and Fransson. An accurate half-life value $T_{\frac{1}{2}}=16.8\mathrm{}\pm 0.2$ nsec was determined from delayed $\beta e^{-}$ coincidences. A total conversion coefficient $\alpha =5.3\mathrm{}\pm 0.3$ was obtained from the relative intensities of the 27.7-keV gamma rays and the 27.4-keV x rays that accompany the isomeric transition in ${\mathrm{Te}}^{129}$. The contributions of these rays were separated by comparing Mössbauer spectra obtained with the 70-min and 33-day activities of ${\mathrm{Te}}^{129}$. Some remarks are made about the relative merits of these activities for use in Mössbauer sources.